Resilient Packet Ring (RPR) is an effective solution for metropolitan area data transport applications. RPR is a Media Access Control (MAC) protocol that operates at Layer-2 of the OSI (Open System Interconnection) protocol stack. RPR provides a ring topology for interconnection among nodes that exchange data with one another. It provides a packet ADM (Add-Drop Multiplexer) architecture and is compatible with Ethernet, SONET (Synchronous Optical NETwork), or DWDM (Dense Wavelength Division Multiplexing) physical layer standards. RPR has a number of characteristics that are responsible for its popularity and are briefly described in a white paper by the RPR Alliance “An Introduction to Resilient Packet Ring Technology”, by Gunnes Aybay, Mannix O'Connor, Kanaiya Vasani and Tim Wu, October 2001. RPR that employs a packet ring technology has the inherent advantage of implementing bandwidth fairness algorithms that are concerned with the allocation of a “fair share” of the ring bandwidth to every customer. Being a packet ring, an RPR can handle multicasting effectively: every node can receive and forward the packet circulating on the ring. An RPR system, in which nodes share a common medium, provides a simplified service model that enables carriers to provide services in a short period of time. An important feature of the RPR is its resiliency to failures such as a fiber cut. The RPR is also self-healing, i.e., a packet that cannot proceed in the original direction due to the failure, can reach the destination by going around the ring in an opposite direction.
RPR protection handles failures within a given RPR with a guarantee that a protection switching will be completed in less than 50 ms. There is a need to provide similar levels of protection for interconnected RPRs.
Such interconnected rings are expected in large metropolitan areas [Ref: Bell Canada RPR Requirements, IEEE 802.17 Interim Meeting, May 2001, by Paul LeBel]. Using a single bridge or router between two interconnected rings leads to a single point of failure: if the interconnection device fails, an inter-ring message cannot be delivered. Robust protection mechanisms equivalent to those provided in SONET are discussed in “SBC Priorities and Objectives for Resilient Packet Ring Development”, by George Young, SBC Technology Resources, Inc., IEEE 802.17, Mar. 12, 2001. Protection requirement for interconnected rings specified in SONET is achieved through a set of double interconnection devices, e.g., GR-1230-CORE and GR-1400-CORE. Dual attachment points on different rings for providing an additional protection path is also addressed in “RPR Requirements, A CLEC Perspective”, by Dave Milliron, IEEE 802.17, RPR Working Group, May 14, 2001 and “NETWORK REQUIREMENTS FOR RPR”, by Italo Busi and Vittorio Mascolo, Alcatel Optics.
However, dual attached interconnections using Layer-2 bridging (or routing) rely on the Spanning Tree Protocol (STP) [IEEE 802.3D STP Standard] or Layer-3 routing protocols (such as OSPF or VRRP) that exhibit large convergence times, typically in the order of seconds.
Accordingly, there is a strong requirement for further improvement of the network protection mechanisms which would achieve protection switching in shorter periods of time that are comparable to the protection switching times specified for a single RPR.